In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size ≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA ≧0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 laser lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography. Efforts have been made for the early introduction of ArF immersion lithography (see Proc. SPIE, Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724, 2003).
Several problems arise when a resist film is exposed in the presence of water. For example, the acid once generated from a photoacid generator and a basic compound added to the resist material can be partially leached in water. As a result, pattern profile changes and pattern collapse can occur. It is also pointed out that water droplets remaining on the resist film, though in a minute volume, can penetrate into the resist film to generate defects.
These drawbacks of the ArF immersion lithography may be overcome by providing a protective coating between the resist film and water to prevent resist components from being leached out and water from penetrating into the resist film (see 2nd Immersion Workshop: Resist and Cover Material Investigation for Immersion Lithography, 2003).
With respect to the protective coating on the photoresist film, a typical antireflective coating on resist (ARCOR) process is disclosed in JP-A 62-62520, JP-A 62-62521, and JP-A 60-38821. The ARCs are made of fluorinated compounds having a low refractive index, such as perfluoroalkyl polyethers and perfluoroalkyl amines. Since these fluorinated compounds are less compatible with organic substances, fluorocarbon solvents are used in coating and stripping of protective coatings, raising environmental and cost issues.
Other resist protective coating materials under investigation include water-soluble or alkali-soluble materials. See, for example, JP-A 6-273926, Japanese Patent No. 2,803,549, and J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 615, 2005. Since the alkali-soluble resist protective coating material is strippable with an alkaline developer, it eliminates a need for an extra stripping unit and offers a great cost saving. From this standpoint, great efforts have been devoted to develop water-insoluble resist protective coating materials, for example, resins having alkali-soluble groups such as fluorinated alcohol, carboxyl or sulfo groups on side chains. See WO 2005/42453, WO 2005/69676, JP-A 2005-264131, JP-A 2006-133716, and JP-A 2006-91798.
As means for preventing resist components from being leached out and water from penetrating into the resist film without a need for a protective coating material, it is proposed in JP-A 2006-48029, JP-A 2006-309245, and JP-A 2007-187887 to add an alkali-soluble, hydrophobic compound as a surfactant to the resist material. This method achieves equivalent effects to the use of resist protective coating material because the hydrophobic compound is segregated at the resist surface during resist film formation. Additionally, this method is economically advantageous over the use of a resist protective film because steps of forming and stripping the protective film are unnecessary.
The ArF immersion lithography systems commercially available at the present are designed such that water is partly held between the projection lens and the wafer rather than immersing the resist-coated substrate fully in water, and exposure is carried out by scanning the wafer-holding stage at a speed of 300 to 550 nm/sec. In the event of such high-speed scanning, unless the performance of the resist or protective film is sufficient, water cannot be held between the projection lens and the wafer, and water droplets are left on the surface of the resist film or protective film after scanning. It is believed that residual droplets cause defective pattern formation.
To eliminate the droplets remaining on the surface of the photoresist or protective film after scanning, it is necessary to improve the flow or mobility of water (hereinafter, water slip) on the relevant coating film. It is reported that the number of defects associated with the immersion lithography can be reduced by increasing the receding contact angle of the photoresist or protective film with water. See 2nd International Symposium on Immersion Lithography, Sep. 12-15, 2005, Defectivity data taken with a full-field immersion exposure tool, Nakano et al.
One exemplary material known to have excellent water slip and water repellency on film surface is a copolymer of α-trifluoromethylacrylate and norbornene derivative (Proc. SPIE, Vol. 4690, p 18, 2002). While this polymer was developed as the base resin for F2 (157 nm) lithography resist materials, it is characterized by a regular arrangement of molecules of α-trifluoromethylacrylate (effective for water repellency improvement) and norbornene derivative in a ratio of 2:1.
When a water molecule interacts with methyl and trifluoromethyl groups, it orients via its oxygen and hydrogen atoms, respectively, and the orientation distance between water and methyl is longer, as discussed in XXIV FATIPEC Congress Book, Vol. B, p 15 (1997) and Progress in Organic Coatings, 31, p 97-104 (1997). A resin having not only water repellent fluorinated units introduced, but also both fluoroalkyl and alkyl groups incorporated is improved in water slip because of a longer orientation distance of water. In fact, a polymer having a regular arrangement of water repellent monomeric units like the above-referred copolymer of α-trifluoromethylacrylate and norbornene derivative is used as the base polymer in a protective coating for immersion lithography, water slip is drastically improved (see US 20070122736 or JP-A 2007-140446).
A material having good water slip performance is also required from the standpoint of productivity. The immersion lithography needs higher throughputs than ever. For improved productivity, the exposure time must be reduced, which in turn requires high-speed scanning operation of the stage. In order to move the stage at a high speed while holding water beneath the lens, it is desired to have a resist material or resist protective film having higher water slip performance.
The highly water repellent/water slippery materials discussed above are expected to be applied not only to the ArF immersion lithography, but also to the resist material for mask blanks. Resist materials for mask blanks suffer from problems including a change of sensitivity during long-term exposure in vacuum and long-term stability after coating. With respect to the control of sensitivity changes in vacuum, an improvement is made by a combination of acid labile groups of acetal and tertiary ester types (U.S. Pat. No. 6,869,744). It is believed that after coating of a resist material, an amine component is adsorbed to the resist film surface whereby the resist varies its sensitivity or profile. A method of modifying the surface of a resist film for preventing adsorption of an amine component to the resist film has been devised.
Hydrophobic surfactants for use in resist protective coatings and resist materials are allegedly effective in overcoming a pattern profile change, referred to as “dark-bright difference,” which is considered problematic in many types of lithography including immersion lithography, dry lithography and EB lithography. The dark-bright difference is a phenomenon that the profile of a line-and-space pattern differs between a bright pattern where a peripheral portion around the pattern is exposed and a dark pattern where a peripheral portion around the pattern is not exposed. When a peripheral portion around the pattern is exposed, the acid generated in the peripheral portion can evaporate during PEB to cover the pattern area, whereby the line pattern undergoes a film slimming. When a peripheral portion around the pattern is not exposed, no acid is supplied from the peripheral portion and inversely, amine evaporates whereby the line pattern takes a bulged top profile. The “dark-bright difference” occurs by this mechanism. The dark-bright difference can be reduced by providing a protective coating on the resist film.
Citation ListPatent Document 1:JP-A S62-62520Patent Document 2:JP-A S62-62521Patent Document 3:JP-A S60-38821Patent Document 4:JP-A H06-273926Patent Document 5:JP 2803549Patent Document 6:WO 2005/42453Patent Document 7:WO 2005/69676Patent Document 8:JP-A 2005-264131Patent Document 9:JP-A 2006-133716Patent Document 10:U.S. Pat. No. 7,455,952 (JP-A 2006-91798)Patent Document 11:JP-A 2006-048029Patent Document 12:JP-A 2006-309245Patent Document 13:JP-A 2007-187887Patent Document 14:US 20070122736(JP-A 2007-140446)Patent Document 15:U.S. Pat. No. 6,869,744Non-Patent Document 1:Proc. SPIE, Vol. 4690, xxix,2002Non-Patent Document 2:Proc. SPIE, Vol. 5040, p724,2003Non-Patent Document 3:2nd Immersion Workshop:Resist and Cover MaterialInvestigation for ImmersionLithography (2003)Non-Patent Document 4:J. Photopolymer Sci. andTechnol., Vol. 18, No. 5, p615,2005Non-Patent Document 5:2nd International Symposium onImmersion Lithography, 12-15Sept., 2005, Defectivity datataken with a full-fieldimmersion exposure tool, Nakanoet al.Non-Patent Document 6:Proc. SPIE, Vol. 4690, p18(2002)Non-Patent Document 7:XXIV FATIPEC Congress Book,Vol. B, p15 (1997)Non-Patent Document 8:Progress in Organic Coatings,31, p97-104 (1997)